[C++20] [Modules] Remove hardcoded path to imported module in BMIs

Close https://github.com/llvm/llvm-project/issues/62707

As we discussed before, we'll forbid the use of implicit generated path
for C++20 modules. And as I mentioned in
https://github.com/llvm/llvm-project/issues/62707, we've emitted a
warning for clang17 and we'll make it a hard error in clang18. And the
patch addresses the decision.